1. Field of the Invention
The invention relates to a method for the detinning of painted tinned sheet waste (tinplate waste) in which the waste is arranged in a bath containing NaOH and subjected to an electrolytic treatment as an anode. By "paint" is included any adherent protective coating of a similar nature to paint.
2. Description of the Prior Art
In the processing of tinplate, 10 to 20% cutting waste is produced. This waste is not processable as scrap for the steel industry because of the high tin content. The same applies to tinplate which is recovered by separation of domestic refuse, or in the separated collection of domestic refuse. Steel scrap of a high quality can however be produced by detinning these waste materials, whilst the recovered tin has a high commercial value.
A conventional method of detinning tinplate waste consists in tipping the material to be detinned loosely into a basket which is then placed in an electrolytic bath. Steel plates are suspended next to the basket, and the basket and these steel plates are connected to a voltage source as electrodes for an electrolytic process, in which the basket (and thus the tinplate itself) and the bath walls serve as the anode, and the steel plates as the cathode, causing the tin from the tinplate waste to be dissolved and deposited on the steel plates. From time to time this deposited tin is removed from the cathodes. After detinning, the detinned material is washed with clean water, and pressed into steel scrap bundles. When processing clean, unpainted tinplate waste it is possible to detin baskets with a content of 80 to 100 kg tinplate in approximately 21/2hours to obtain a residual tin content of less than 0.2% Sn.
In order to render tinplate suitable for many applications, one or more coats of paint are nowadays very often applied to it. These coats of paint provide additional protection, but make detinning more difficult because the layer of tin is screened off from the electrolytic bath.
Various measures have been proposed for reducing the effect of the paint on the detinning process. Thus it has been proposed to give the painted tinplate waste a heat pretreatment, in which the paint is burnt off. It has also been proposed intensively to damage the coat of paint mechanically so that the electrolyte gains access to the underlying tin layer. Both methods suffer from the most serious disadvantage that they give rise to considerable and undesirable contamination of the electrolytic bath as a result respectively of ash residues deriving from the paint, and of flakes of paint that have come loose in the mechanical pretreatment.
It is conventional to wash the detinned waste with water before processing it further as scrap. In the case of highly contaminated detinning baths, conventional washing leads to a relatively high residual tin content in the scrap, which is undesirable. On the other hand, if additional washing is carried out, then more tin may be lost in the washing water.
It has also been proposed to attack the coat of paint chemically first, for example by means of polyethylene glycol ether, or by means of methyl alcohol. One drawback to this method consists in that the chemicals used either fail to attack all the known paints used, or are so toxic that the entire process must be carried out in closed vessels, thereby increasing costs. In most cases this process must also be carried out at a high temperature.
More specifically, FR-A No. 1 496 440 describes the use of polyethylene glycol ethers to remove paint prior to, for example, electrolytic detinning. Paint removal is apparently aimed at, and it is clear that the glycolic ethers achieve this. FR-A No. 1 000 856 also proposes the use of soap solutions containing sodium hydroxide to achieve rapid removal of paints by brief immersion followed by a few seconds of electrolysis. The aim is to avoid any attack on the metal under the paint.
Detinning by non-electrolytic solution of the tin followed by recovery of the tin from the solution so produced has also been proposed. Specifically, U.S. Pat. No. 1,511,590 (published 1924) describes a process in which tin is recovered from tinplate by (1) removing for example paint by means of a weak solution of caustic, (2) dissolving the tin by means of a hot oxidizing solution of alkali, (3) crystallizing out the resultant sodium stannate and (4) electrolyzing a solution of the redissolved sodium stannate to obtain tin metal. This is a complex multi-stage process.